This invention relates to the field of nuclear radiation measurement and is an improvement over the apparatus and method for radiation measurement disclosed and taught in the co-pending U.S. patent application of Elmo J. DiIanni, Harold J. Cooley, Michio Fujita and Charles Victor Noback, Ser. No. 562,985 filed Dec. 19, 1983, and assigned to the same assignee as the present patent application.
The co-pending application fully describes the background of the invention and the known prior art, and shows the derivation of the governing equations upon which the apparatus and method of that application are based using a Geiger-Mueller (G-M) tube radiation detector, such equations being EQU R=K/T
where:
R is radiation field strength PA1 T is the time to first strike after G-M tube enablement PA1 K is a proportionality constant ##EQU1## where: R.sub.T is the radiation intensity over a defined time period PA1 .SIGMA.COUNTS is the number of strikes occurring during the defined time period PA1 .SIGMA.T.sub.ON is the G-M tube "on time" to register each strike during the defined time period PA1 (a) Keep count of total number of strikes PA1 For each new strike: PA1 (b) Keep total of "time to strike" i.e. (total time G-M tube is ON). PA1 For each new "time to strike": PA1 (a) Adjust TOTAL TIME for errors in "time to strike" measurements PA1 (b) RATE=TOTAL STRIKES/ADJUSTED TIME.times.SCALE FACTOR PA1 (a) the need to manually calibrate the time error constant used to compensate for the time to strike error. This calibration requires an especially large radiation source and is frequently impractical. PA1 (b) errors resulting from slow drift in the time to strike error during use. PA1 (c) external hardware switches required to specify the time error constant. PA1 HISTOG EVENT COUNT in bin 4.fwdarw.HISTOG EVENT COUNT in bin 4+1 PA1 NEW MAIN EVENT COUNT=OLD MAIN EVENT COUNT+1 PA1 NEW MAIN EVENT ACCUM. TIME SUM=OLD MAIN EVENT ACCUM. TIME SUM+EVENT TIME
As disclosed in the co-pending application, the G-M detector is enabled by raising the bias voltage across the detector up into its active region and measuring the elapsed time interval thereafter to the occurrence of the first strike. Since the reciprocal of this time interval is proportional to the radiation field strength, as shown by the above equations, all information necessary to determine the field strength (R, R.sub.T) has been obtained. A constant wait time is employed after each strike to assure that the G-M tube has fully recovered, and the G-M tube is then enabled and the process is repeated. Because of the random nature of nuclear phenomena, even though each timed interval to strike contains all of the necessary information to calculate the radiation field strength, the confidence level that any given measurement is an accurate representation of the true average field strength is low. Many measurements are therefore taken and combined for high confidence factors.
In the method according to the co-pending application an estimate of rate (field strength) is made at regular intervals (every two seconds). This is done in two stages. First, new data is collected which contains information about the number of strikes and the "times to strike". Then a new estimate of rate is calculated. The calculation is done in several steps. First, the total accumulated time is adjusted to correct for time to strike errors. Then, the total number of counts is divided by the total accumulated time. Following this a scale factor is then applied to compensate for the "N" factor of the Geiger-Mueller tube.
In high fields, errors in the measurement of time to strike become a problem. These errors creep in from several sources. One error is caused by measuring time in discrete steps which limits the time resolution. Other errors tend to inflate the value of the measured time to strike. Among the reasons for this increase in time are G-M tube turn-on delays and circuit propagation delays. The error can drift slowly over time and is somewhat different from one unit to the next. To help correct for this problem a time error constant is subtracted from each time to strike measurement. The value of this constant is read from external switches which can be set by the user. After the accumulated time has been adjusted for time to strike errors, calculations proceed as discussed above (total counts are divided by total accumulated time).
This method is summarized as follows.
Collect data:
TOTAL STRIKES=TOTAL STRIKES+1 PA2 TOTAL TIME=TOTAL TIME+"TIME TO STRIKE" PA2 (1) Read in TIME ERROR constant from switches. PA2 (2) ADJUSTED TIME=TOTAL TIME-(TOTAL COUNTS.times.TIME ERROR)
Calculate RATE:
The foregoing described method, and the apparatus for implementing it, is a substantial improvement over the prior art but nevertheless is still subject to certain problem areas, particularly in very high radiation fields where the mean time to strike is very short, for example in the one microsecond region. These problems are: